Examples of currently-used nuclear medicine diagnosis apparatus of this type, i.e., an ECT (Emission Computed Tomography) apparatus, include a PET (Positron Emission Tomography) apparatus. The PET apparatus reconstructs a tomographic image of a subject only when a plurality of detectors simultaneously detects two gamma-rays generated through annihilation of a positive electron (Positron). Specifically, radiopharmaceutical containing positron-emitting radionuclide is administered to the subject. Then paired annihilation gamma-rays of 511 Key emitted from the administered subject are detected with the detectors each formed by many detector elements (e.g., scintillators). Thereafter, when two gamma-rays are detected with two of the detectors within a certain time period, the gamma-rays are detected “coincidently”. Moreover, a position where the paired annihilation gamma-rays are generated is identified to be on a line connecting the two detectors. Then information on such coincidence is stored for reconstruction to obtain a positron-emitting radionuclide image, i.e., a tomographic image.
The PET apparatus includes a plurality of detectors disposed in a ring shape so as to surround the subject for enhanced detection sensitivity. Such arrangement that the detectors are disposed in a ring shape is referred to as a detector ring. In addition, with a site-specific PET apparatus that captures a part of the subject, detectors are disposed closely to the subject so as to surround only the part of the subject. This achieves more enhanced detection sensitivity.
Examples of the site-specific PET apparatus include a mammography PET apparatus. See Patent Literature 1. FIGS. 11(a) and 11(b) each illustrate a vertical sectional view of a currently-used mammography PET apparatus 101 (Patent Literature 1). The mammography PET apparatus 101 includes a supporting unit 102 that supports a subject M in a prone position or in a seated position, an opening 111 formed on a supporting face 104 of the supporting unit 102, and a detector ring 105 that is disposed below the supporting face 104 of the supporting unit 102 and surrounds a breast b of the subject M inserted into opening 111.
The mammography PET apparatus 101 of Patent Literature 1 has the following drawback. That is, breast cancer mostly occurs at a root of the breast b of the subject M, i.e., around a chest wall of the subject M. Accordingly, the breast b of the subject M is desired to be inserted more deeply into a field of view of the detector ring 105. On the other hand, a single mat is disposed on the supporting face 104 as a cushion. This mat prevents more deep insertion of the breast b of the subject M into the field of view. As a result, a chest of the subject M should be pressed toward the detector ring 105 with an own force of the subject M using a handle.
Moreover, when no mat is disposed, the subject M contacts directly to the hard supporting face 104. Accordingly, significant pain induces on sternum of the subject M or a site of the subject M contacting to an outer edge of the opening 111, for example.
Consequently, Patent Literature 1 has suggested the following. Specifically, as illustrated in FIGS. 11(a) and 11(b), the mammography PET apparatus 101 includes a chest mat 133 disposed around the opening 111 in the supporting unit 102 for supporting the chest of the subject M, and an outer mat 131 formed outward of the chest mat 133 for supporting the subject M. That is, two types of mats 131 and 133 are disposed side by side for different sites. The chest mat 133 is formed of a material softer (having a greater sinking ratio) than the outer mat 131. Upon support of the subject M with the chest mat 133 and the outer mat 131, the chest mat 133 sinks more deeply than the outer mat 131. As a result, a weight of the subject M concentrates at the chest of the subject M, i.e., around the breast b. This facilitates pressing of the chest of the subject M against the detector ring 105. In addition, since the chest mat 133 supports the subject M, pain induced on the subject M is alleviated.